line has undefined or infinite slope (see below).

If two points of a road have altitudes y₁ and y₂, the rise is the difference (y₂ - y₁) = ?y. Neglecting the Earth's curvature, if the two points have horizontal distance x₁ and x₂ from a fixed point, the run is (x₂ - x₁) = ?x. The slope between the two points is the difference ratio:
:

Through trigonometry,

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